1. Field of the Invention
The present invention relates to a novel perfluoroalkyl vinyl ether compound, a process for preparing a copolymer by using the perfluoroalkyl vinyl ether compound, and an optical plastic material comprising a copolymer prepared by the process. More particularly, the present invention relates to a cyclic perfluoroalkyl vinyl ether having a particular molecular structure, a process for preparing a copolymer by copolymerizing the perfluoroalkyl vinyl ether compound with a common fluorinated olefin in the presence of a perfluorinated radical initiator, and an optical plastic material comprising a copolymer prepared by the process, and optionally a dopant.
2. Description of the Related Art
In recent years, a number of optical plastic materials have been used as media for light transmission in the field of automobiles, OA machines and various sensors. For example, plastic optical fibers (POFs) are widely used as optical fibers for short-distance communication in place of quartz glass. Plastic optical fibers are largely classified into graded refractive index (GI) type POFs and stepped refractive index (SI) type POFs. Generally, methylmethacrylate, styrene, carbonate, norbornene resins and the like are used as cores or clads of optical fibers. These resins are polymers having C—H bonds in their molecular structure. When the polymer materials are used as materials for optical fibers, stretching or deformation vibration takes place due to the presence of hydrogen atoms in the C—H bonds. As a result, the polymer materials absorb light of a characteristic wavelength corresponding to the vibration and light transmittance is therefore decreased. This is a main cause of optical loss. For instance, polymethylmethacrylate is evaluated to have a theoretical absorption loss of 105 dB/km for a 650 nm light source and 10,000 dB/km for a 1,300 nm light source, due to C—H bonds contained in the compound.
It has been found that when the hydrogen atoms in the polymers are replaced with fluorine atoms, no absorption loss substantially takes place for a 650 nm light source and a theoretical absorption loss is only about 1 dB/km between sixth and seventh overtones of the C—F bonds for a 1,300 nm light source. For this reason, extensive research has been undertaken on the use of fluorine polymers having C—F bonds as materials for optical fibers.
For example, Japanese Patent Laid-open No. Hei 8-334634 discloses poly(heptafluoro-1-butene-trifluoro-vinylether) (hereinafter, referred to as ‘PBVE’), as a completely fluorinated optical plastic material, represented by the following formula:

This polymer is prepared by homopolymerizing perfluoro(2,2-dimethyl-1,3-dioxole) having an allyl cyclic structure containing fluorine atoms in the main chain of the monomer, or copolymerizing the monomer with tetrafluoroethylene or hexafluoropropylene.
Since the polymer PBVE is prepared from the monomer having an allyl cyclic structure containing fluorine atoms in the main chain, it contains no C—H bonds. In addition, it is known that PBVE has an amorphous structure and a Tg of 108° C. Further, the polymer PBVE is commercially available under the brand name of “CYTOP” from Asahi Glass Co., Japan.
In addition to PBVE, Asahi Glass Co. proposed fluorinated polymers as materials for a GI (graded index) plastic optical fiber, represented by the following formulae:

wherein 1 is a number of 0 to 5, m is a number of 0 to 4, n is a number of 0 to 1, and 1+m+n is a number of 1 to 6; o, p and q are each independently a number of 0 to 5, and o+p+q is a number of 1 to 6; R, R1 and R2 are each independently F or CF3; and X1 and X2 are each independently F or Cl.
In order to produce POFs, particularly GI type POFs having a refractive index gradient using the above perfluorinated polymers, the use of dopants is required (e.g., low molecular weight compounds or high molecular weight compounds of oligomers or higher polymers containing no C—H bonds in their molecular structure). Additional requirements for the dopants are excellent compatibility with host polymers and a refractive index difference from the host polymers above a predetermined level. Many low molecular weight dopants are known, for example, halogenated aromatic hydrocarbons containing no C—H bonds. Among these hydrocarbons, halogenated aromatic hydrocarbons containing only fluorine atoms as halogen atoms and halogenated aromatic hydrocarbons containing fluorine atoms and other halogen atoms are preferred in terms of high compatibility with host polymers. As high molecular weight or oligomeric dopants, perfluorinated polymers having a refractive index different from host polymers are known, for example, perfluorinated polymers containing only fluorine atoms as halogen atoms, and perfluorinated polymers containing fluorine atoms and other halogen atoms.
The production of GI type POFs using a perfluorinated polymer such as CYTOP is achieved by first fabricating a preform for a plastic optical fiber (POF) having a refractive index gradient in a polymer by using a dopant, followed by thermal drawing. At this time, the fabrication of the preform is possible by various methods (see, e.g., Japanese Patent Nos. 10-268146 and Japanese Patent Laid-open No, Hei 8-334534): For example,
1) A perfluorinated polymer is melted; a dopant or a perfluorinated polymer containing the dopant is fed into the central portion of the molten polymer; and, the dopant is diffused so as to be molded into a preform.
2) A perfluorinated polymer prepared by melt-spinning; drawing is used to form a central rod; and a dopant or a perfluorinated polymer containing the dopant is repeatedly dip-coated onto the rod.
3) A hallow is formed in a perfluorinated polymer using a rotating glass tube; a dopant or a perfluorinated polymer containing the dopant is filled into the perfluorinated polymer tube; and, the resulting structure is rotated at a low speed.
4) A homogeneous mixture of a perfluorinated polymer and a dopant-or a mixture obtained by uniformly mixing the perfluorinated polymer and the dopant in a solvent and evaporating the solvent only-is thermally drawn or melt-extruded to produce a fiber; and, the fiber is contacted with an inert gas under heating to form a refractive index gradient.
5) A rod or fiber consisting of a perfluorinated polymer is formed; a dopant having a refractive index lower than the perfluorinated polymer or a perfluorinated polymer containing the dopant is coated on the rod or fiber; and, the coated structure is heated to diffuse the dopant, thereby forming a refractive index gradient.
6) A high refractive index polymer and a low refractive index polymer at various mixing proportions are heat-melted or mixed in a solvent; and the obtained mixtures are diffused by extruding in a multilayer, thereby producing a fiber having a refractive index gradient.
7) A stepped or multi-stepped preform is fabricated using a perfluorinated polymer and a dopant, and the dopant is diffused at the interface between the steps to produce a GI type optical fiber.
In accordance with the above-mentioned methods, Asahi Glass Co., Japan and professor Koike developed GI type POF (commercial name: “Lucina”) having excellent optical properties at the level of an attenuation of 80 dB/km and a bandwidth of 3 Gbps/100 m. It was developed so as to apply to office LAN and optical interconnection. “Lucina” exhibits a very low optical loss, a large transmission capacity at various wavelengths, and excellent moisture resistance. However, the present inventors have found problems with “Lucina” in that the optical properties are non-uniform along the length of the fiber, and the dopant used to control the refractive index gradient acts as a plasticizer. This greatly deteriorates the thermal properties of the base resin, damaging the long-term reliability. Particularly, the refractive index gradient is deformed, decreasing the transmission capacity.
Accordingly, prior art perfluorinated polymers are not suitable to produce POFs for access networks, office networks, automobiles, military purposes and aircraft, which require excellent heat resistance and long-term reliability. When a dopant such as CTFE (chlorotrifluoroethylene) is combined with “CYTOP”, a non-crystalline, completely fluorinated homopolymer having a Tg of about 108° C., to produce a plastic optical fiber, it functions to greatly lower the Tg of the host polymer. This combination of the dopant and CYTOP reduces the Tg of the final POF to less than 90° C. Since the reduction in Tg decreases time and temperature stability, the use of “CYTOP” is limited in its application to POFs.
Examples of other fluorine-based polymers include Teflon AF (Dupont), a copolymer of 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole and tetrafluoroethylene, represented by the following formula:

wherein n is a real number of 1 or less.
However, according to a report by the International Plastic Optical Fiber Conference (2001), Teflon AF was reported to have a high optical loss.
In order to effectively produce rod lenses, optical waveguides, optical decouplers, wavelength multiplexers and wavelength demultiplexers, optical attenuators, optical switches, optical isolators, light transmitting modules, light receiving modules, couplers, optical detectors and optical integrated circuits as well as POFs, using optical plastic materials, the optical plastic materials must possess the following properties: they must not exhibit light scattering; they must be substantially transparent over a very broad range of wavelengths; and, they must be excellent in various physiochemical properties including heat resistance. No optical materials satisfying these requirements have hitherto been reported.
There is therefore a need in the art for an optical plastic material that exhibits the following properties: little or no light scattering; substantially transparent in the UV (wavelength: 200˜400 nm) and near IR (wavelength: 2,500 nm or shorter) regions; excellent heat resistance, therefore being stable even in the presence of a dopant; and, excellent chemical resistance, moisture resistance and flame retardancy.
The present inventors have earnestly and intensively conducted research to develop an optical plastic material satisfying the above-mentioned requirements. As a result, the present inventors have found that a polymer prepared by the copolymerization of a cyclic perfluoroalkyl vinyl ether having a particular molecular structure and a perfluorinated olefin monomer (for example, tetrafluoroethylene) in a chlorofluorinated organic solvent in the presence of a perfluorinated radical initiator has the following properties: it is substantially transparent in the UV and near IR regions; it has little or no optical loss upon light penetration; it has excellent heat resistance, chemical resistance, moisture resistance and compatibility with conventional dopants; and, it maintains excellent heat resistance even in the presence of a dopant. The refractive index gradient formed by a dopant in the polymer is not deformed despite time passage and varying temperatures and is stably maintained. The present invention is based on these findings.